DIY Doctor

Is this the correct way to install Celotex?

Postby TomBowyer » Thu Oct 25, 2012 1:41 pm

I wonder if someone could help me please;

Im planning on installing rigid board insulation (e.g celotex/kingspan) on the inside of my house (its a 1920s bungalow with thin 15cm timber frame walls). Would you mind advising on the following?

1. I assume I can just leave the existing walls and plaster completely alone and just build an internal stud wall which I can fill with insulation. If so do I just attach the new studs to the existing studs through the plaster(using some very long screws/nails?) or do I need to remove all the old plaster to reveal the existing studs?

2. Do I need to leave an air gap on either side of the celotex/kingpspan, or should it be flush against the plasterboard?

3. Do I need to put some form of water proof barrier between the celotex/kingspan and the plasterboard, and if so which side (I think its the internal side right?)

4. If I wish to install 100mm of celotex what would be the total size of the new stud wall (i.e how much of the room would I loose?)

Thanks very much!
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Simply Build It

Postby welsh brickie » Fri Oct 26, 2012 2:28 pm

if its timber stud then I would remove the plasterboard,its easier.Remember you will have to alter the electrical back boxes aswell,and the window boards,skirting, architraves.The moisture barrier will be on the other side of the side of the stud so it wont affect the alteration.
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Postby Perry525 » Mon Nov 19, 2012 10:05 am

You have 2x6 inch frame, 50x150mm studs, no insulation and

plasterboard on the inside?

You propose adding new studs to make the frame, how thick?

And you want to add 100mm thick Celotex to the inside of the

resulting frame.

A better way.

If you fit four inches/100mm thick polystyrene between the

existing studs making it flush with the room side of the studs

and cutting to a push tight fit, leaving no gaps, then fit another

layer of Celotex/Quin/Kingspan or similar foil faced foam

board over the studs, you will have a very warm wall.

The inner layer of foam board is to stop the studs from

becoming a heat bridge between the warm plasterboard and

the outside. The foil side enables you to stick a wide plastic

adhesive over the gaps between the foam board, this is

important as it is needed to stop the movement of water

vapour from the room to the frame, thus avoiding

condensation inside the frame and possible wood rot.

Probably a 50mm board with a foil face will make for easy

water vapour proofing and this will provide a good level of

insulation. Another point you can buy 75mm plasterboard

screws made to fit this type of instalation. Celotex/Kingspan

products are expensive.

In order this would mean, skimmed plasterboard, 50mm foam

board fitted on the inside of the frame to stop the frame from

bridging the insulation, 100mm polystyrene between the

studs, a water proof barrier to stop the rain from entering the

frame and the exterior finish.

You then need to consider the ceilings and floors, the ceilings

need to be of the same insulation standard, not so much heat

escapes through the floor, however its nice to have warm feet

and to slow the loss of heat through the floor.
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